This report will describe recent findings on the mechanism of action and properties of a new mammalian mitochondrial transhydrogenase which catalyzes the transfer of hydrogen from a hydroxyacid to an oxoacid. Quantitative transfer of deuterium from the hydroxygl bearing carbon of a hydroxyacid to the alpha-carbon of alpha-ketoglutarate has now been demonstrated. Purification and preliminary determination of the molecular weight of this enzyme are being carried out.